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Former US President Barack Obama has criticised Donald Trump's weak response to neo-Nazis after the Charlotesville riots and warned of "dangerous, extraordinary times."

Obama said during a speech at the University of Illinois on Friday, where he accepted an award for ethics in government, that US President Trump is "capitalising on resentments that politicians have been fanning for years."

He specifically called out Trump's response to the violence in Charlotesville, Virginia in 2017, asking the crowd: "How hard can that be, saying that Nazis are bad?"

"We're supposed to stand up to discrimination, and we're sure as heck supposed to stand up clearly and unequivocally to Nazi sympathisers."

In a now infamous comment following the clash of white supremacists and counter-protesters, which saw counter-protester Heather Heyer killed when a Nazi-sympathiser drove a car into a crowd, Trump said there were "some very fine people on both sides".

In response to Obama's rebuke, Trump told a rally in North Dakota hours later "I'm sorry, I watched it, but I fell asleep. I found he's very good, very good for sleeping."

Obama also said Trump was a "symptom, not the cause" of divisions in America, instead saying it was the result of a rapidly changing world and growing inequality, reports news.com.au.

US President Donald Trump speaks to the press aboard Air Force One on September 7, 2018. Photo / AP

"Sometimes the backlash comes from people who are genuinely, if wrongly, fearful of change," he said.

"More often it's manufactured by the powerful and the privileged who want to keep us divided and keep us angry and keep us cynical because it helps them maintain the status quo and keep their power and keep their privilege.

"It did not start with Donald Trump. He is a symptom, not the cause. He's just capitalising on resentments that politicians have been fanning for years, a fear and anger that's rooted in our past but it's also born out of the enormous upheavals that have taken place in your brief lifetimes."

"They're undermining our alliances, cosying up to Russia. What happened to the Republican Party? Its central organising principle in foreign policy was the fight against communism, and now they're cosying up to the former head of the KGB."

Obama called on people to vote in America's midterm November election, saying "our democracy depends on it."

He said the country is at a pivotal moment in its history and "the consequences of any of us sitting on the sidelines are more dire" than in prior elections.

"Because in the end, the threat to our democracy doesn't just come from Donald Trump or the current batch of Republicans in congress or the Koch brothers and their lobbyists or too much compromise from Democrats or Russian hacking," he said.

"The biggest threat to our democracy is indifference. The biggest threat to our democracy is cynicism. Cynicism led too many people to turn away from politics and stay home on election day."

The speech is Obama's first big step into the campaign for the midterm elections. His advisers say it's a preview of the case he'll make throughout the fall campaign season.

After spending much of his post-presidency on the political sidelines, Obama has several events scheduled in coming days where he'll campaign for Democrats. He'll next travel to California for an event with seven House candidates in Orange County. Next week he'll be in Ohio to campaign for Senator Richard Cordray and other Democrats.